The invention relates to a rubber boot adapted to cover a joint portion interposed between a drive shaft and wheel shafts of an automobile or other motor vehicle.
Wheel shafts of an automobile are rotated through a drive shaft and an universal joint. As is well known, a joint box which encloses a universal joint and an outer surface of one end of the drive shaft are covered by a boot formed of a flexible material such as rubber to prevent dust or particulate materials from entering the gear box and to confine grease so as to lubricate the universal joint.
A conventional boot structure is shown in FIGS. 1 and 2 wherein a bellows-like boot has a small diameter portion A adapted to be secured to a drive shaft Xa and a large diameter portion B adapted to be secured to a joint box Xc. Between the large and small diameter portions, a plurality of annular ridges 1, 2, 3 and 4 are provided at equal intervals l. These ridges are radially oriented in the direction perpendicular to the axial direction of the drive shaft with the diameter of the ridges gradually increasing toward the larger diameter portion. Between respective neighbouring ridges, valleys 12, 13 and 14 are formed. Further, a first valley 11 and fifth valley 15 are provided immediately adjacent the small and large diameter portions, respectively.
For simplicity, the bellows of the boot is divided into a first portion adjacent to the small diameter portion A and a second portion adjacent to the large diameter portion B by the third valley. The first portion includes the first and second ridges 1, 2, first and second valleys 11, 12, first and second planers 1a, 2a facing toward the small diameter portion A, and opposite planes 1b, 2b facing toward the large diameter portion B. These first and second ridges 1, 2 are formed to have ridge angles .theta..sub.1a and .theta..sub.2a, which are defined by the first plane 1a and a first diametrical plane P.sub.1 and by the second plane 2a and a second diametrical plane P.sub.2, respectively, of 0.degree. to 6.degree., and angles .theta..sub.1b and .theta..sub.2b, which are defined by the first opposite plane 1b and the first diametrical plane P.sub.1 and by the second opposite plane 2b and the second diametrical plane P.sub.2, respectively, of 30.degree. to 38.degree..
The second portion includes third to fourth ridges 3, 4, fourth and fifth valleys 14, 15, third and fourth planes 3a, 4a facing toward the small diameter portion A, and opposite planes 3b and 4b facing toward the large diameter portion B. These third and fourth ridges 3, 4 are formed so as to have ridge angles .theta..sub.3a and .theta..sub.4a, which are defined by the third plane 3a and a third diametrical plane P.sub.3 and by the fourth plane 4a and a fourth diametrical plane P.sub.4, respectively, of 11.degree. to 17.degree., and angles .theta..sub.3b and .theta..sub.4b, which are defined by the third opposite plane 3b and the third diametrical plane P.sub.3 and by the fourth opposite plane 4b and the fourth diametrical plane P.sub.4, respectively, of 45.degree. to 70.degree..
As shown in FIG. 2, in case the joint angle .alpha. defined between the drive shaft Xa and the wheel shaft Xb should be 30.degree. or more, an interior angle side of the boot is subject to a compression force P adjacent the large diameter portion along the direction shown by a first arrow and to tensile force E adjacent the small diameter portion A along the direction shown by a second arrow while an exterior angle side of the bellow is subject to a tensile force E.sub.1 adjacent the large diameter portion B along the direction shown by a third arrow and to a compression force P.sub.1 adjacent the small diameter portion A along the direction shown by a fourth arrow. At the portion of the bellows adjacent to the large diameter portion on the interior angle side, corrugations are in line contact with one another between the third opposite plane 3b and fourth plane 4a as indicated at 6 due to an undesirable folding of the bellows. This undesirable folding occurs when .theta..sub.3b, .theta..sub.4b are larger than .theta..sub.3a, .theta..sub.4a and when the width of 3b, 4b is larger than width of opposite planes 3a, 4b in combination with the compression force P.
If such line contact repeatedly takes place by the boot being rotated together with the drive shaft Xa, the bellows may be broken due to wear by normal rotation of the drive shaft Xa (for example, 400 r.p.m. 40 km/hr.). Further, there is a chance that the fifth valley 15 immediately adjacent to the large diameter portion will become entangled in the gap between the drive shaft Xa and the joint box Xc thereby also breaking the bellows. Furthermore, as mentioned the above, since the distance between neighbouring ridges is approximately equal, the boot is expanded in the radial direction thereof due to centrifugal force of the grease contained therein at high rotation speeds of the drive shaft (for example, 1800 r.p.m.), resulting in interference or rubbing between the radially expanded boot and neighbouring mechanical parts.